It's a quite old question, but I'll add my answer anyway, as the method I suggest didn't appear in the others
\begin{tabular}{|c|c|c|}
\hline
Foo bar & \begin{tabular}[x]{@{}c@{}}Foo\\bar\end{tabular} & Foo bar \\
\hline
\end{tabular}
where x is either t, c, or b to force the desired vertical alignment.
In case this is needed in more than a couple of places, it's better to define a command
\newcommand{\specialcell}[2][c]{%
\begin{tabular}[#1]{@{}c@{}}#2\end{tabular}}
so the table line before can be one of
Foo bar & \specialcell{Foo\\bar} & Foo bar \\ % vertically centered
Foo bar & \specialcell[t]{Foo\\bar} & Foo bar \\ % aligned with top rule
Foo bar & \specialcell[b]{Foo\\bar} & Foo bar \\ % aligned with bottom rule
More variations are possible, for instance specifying also the horizontal alignment in the special cell.
Notice the @{} to suppress added space before and after the cell text.